Chapter 4 - Results for C/H Systems

4.1 Introduction

 

As demonstrated in the previous chapter, the technique of molecular beam mass spectrometry is able to provide quantitative measurements of both stable and free radical gas-phase species under conditions typical in the diamond CVD process. Due care has to be taken, however, in the data reduction procedures because the overall system sensitivity is critically dependent on the local temperature, pressure and composition of the gas being sampled. Such variations in the sampling efficiency can be offset by performing simple empirically-based correction procedures. Equipped with a sensitive gas-phase analysis technique and the necessary data reduction procedures for the characterization of a CVD environment, gas phase composition measurements were performed for a variety of source gas mixtures. Extensive work has already been done on the effects that different hydrocarbon precursors (See Table 3.2) have on the CVD diamond growth mechanism by comparing the variation in the composition of the stable gas phase species and CH3 radicals as a function of filament temperature.3.1,3.2,3.14 These results are not discussed in detail in this chapter, though the overall conclusions drawn for these hydrocarbon/H2 systems have been included, in order to enable the reader to identify and understand the changes that occur when other gas phase systems are used, such as C/H/Cl, C/H/N and C/H/P systems (which will be discussed fully in their respective chapters).


4.2 Cracking patterns of CH4

 

Fig.4.1

Figure 4.1 shows how the signal intensities of methane and its dissociation products vary as a function of electron energy in the ionization chamber of the mass spectrometer. By the same linear interpolation method described in Section 3.5 (a), the measured ionization potential (I.P.) for CH4 and the appearance potentials (A.P.s) for the different species observed (CH3, CH2 and CH) were obtained and compared with literature values (Table 4.1).

 

Species

Measured I.P.

(eV)

Literature Value

(eV)3.21

Measured A.P.

(eV)

Literature Value

(eV)3.21

CH4 (m/e=16)

12.4 0.2

12.80

-

-

CH3 (m/e=15)

-

9.96

13.9 0.3

14.3

CH2 (m/e=14)

-

-

17.6 2.4

16.5

CH (m/e=13)

-

-

23.8 1.5

23.4

 

Table 4.1. I.P.s and A.P.s of methane and its observed dissociation fragments.

 

The A.P.s of the species observed after electron bombardment of methane relate to the energy required for the following processes:3.21

 

A.P. (CH3): CH4 CH3+ + H + e-

A.P. (CH2): CH4 CH2+ + H2 + e-

A.P. (CH): CH4 CH+ + H + H + H + e-


Inspection of Figure 4.1 shows that dissociative ionisation of CH4 produces mainly methyl ions, with only small amounts of CH2+ and CH+ ions are formed. This may be related to the number of C-H bonds that need to be broken.

 

4.3 Cracking patterns of C2H2

 

Figure 4.2 shows how the signal intensity of acetylene and its dissociation products vary as a function of electron energy in the ionization chamber of the mass spectrometer. The I.P. of acetylene and A.P.s of the dissociation products are summarized in Table 4.2, and compared with the literature values.

Fig.4.2

 

Species

Measured I.P.

(eV)

Literature Value

(eV)3.21

Measured A.P.

(eV)

Literature Value

(eV)3.21

C2H2 (m/e=26)

11.1 0.5

11.41

-

-

C2H (m/e=25)

-

-

17.9 0.5

17.8 0.5

CH (m/e=13)

-

-

22.6 0.4

22.3 0.5

 

Table 4.2. I.P.s and A.P.s of acetylene and its observed dissociation fragments.

 

Inspection of Figure 4.2 reveals that acetylene contributes by far the greatest signal. Only trace amounts of the C2H and CH species were observed at high electron energies (above 22 eV).

 

The A.P.s of C2H and CH arise from the following processes:3.21

A.P. (C2H): C2H2 C2H+ + H + e-

A.P. (CH): C2H2 CH+ + C + H + e- (or CH+ + CH + e-, neither was given)

4.4 Gas phase composition as a function of filament temperature for 1% CH4 in H2

 

Figure 4.3 shows how the concentration of the major hydrocarbon species [CH4 (m/e =16), C2H4 (m/e =26) and C2H2 (m/e =28)] as well as CH3 radicals change as a function of filament temperature measured 4 mm from the filament for an initial CH4/H2 feedstock ratio of 1%. Note that the total carbon balance, defined as (total C fraction measured)/(C fraction in the feed gas), and shown as an upside down triangle in Figure 4.3, decreases as the filament temperature increases, because of the thermal diffusion effects described in Section 3.5 (e).

Fig.4.3

 

The main chemical conversion occurring in the chamber is that of methane to acetylene as the filament temperature is increased, a reaction that is initiated by the reaction of H atoms with methane to produce methyl radicals:

 

CH4 + H CH3 + H2 (4.1)

 

Methyl recombination followed by successive H abstraction yields acetylene. As the filament temperature increases, the increasing H atom concentration drives the equilibrium from CH4, through C2H6 and C2H4 to C2H2. Due to its transient nature (and thus very low steady-state concentration) under conditions rich in H atoms, no C2H6 was detected in the gas phase. Quantitative measurements of the absolute concentrations of methyl radicals were made simultaneously and are displayed in Figures 4.3. An increase in the filament temperature results in higher [CH3] which is mirrored by increased [C2H2]. Therefore the CH3 radical is an essential intermediate in the formation of acetylene.


4.5 Discussion of errors

 

Shown in Figure 4.4 are the errors that are inherent in the calculated species concentrations for the various stable hydrocarbon species (and CH3 radicals) monitored. These values were obtained using standard combination of errors techniques, in which the error in the derived parameter (in this case the species concentrations) is determined from the measured parameters (i.e. the MBMS signal intensities of each of the measured species). Calculations for both stable and free radical species were semi-automated using a spreadsheet program (ASEASYAS). Similar calculations were performed for all the other gas phase systems, including C/H/Cl, C/H/N and C/H/P systems. However, since the gas phase species lie very close to each other on the graphs, their concentrations are presented with no error bars for clarity. The errors shown in Figures 4(a)-(d) are representative of the magnitude observed for all subsequent MBMS experimental runs.

Fig.4.4a

Fig.4.4b

Fig.4.4d

Fig.4.4d

 

It is worth considering the sources of other uncertainties when interpreting the MBMS data. The factors affecting the error that are inherent in the calculated species mole fractions not only include (1) the sensitivity of the MS and (2) the signal/noise ratio, but also (3) changes in the kinetic energy of the electron bombardment (which will relate to the age of the cathode filaments) and (4) filament/sampling cone distances. These factors will inevitably influence the uncertainties in the run to run reproducibility of the MS system, and hence the accuracy of the measured species mole fractions.

 

There are also errors that may be inherent in the measured values of the filament temperature. Such errors will arise from uncertainty in the positioning of the pyrometer and the emissivity of the filament, which reflects on the age of and history of the filament. In the latter case, for example, the emissivity of the tantalum filament may be different to that of tantalum carbide which is formed when the filament is carburised by CH4. Throughout the MBMS experiments conducted in this work the tantalum filament had been previously seasoned or carburised with the source gas mixture later being examined.

 

4.6 Gas composition as a function of filament temperature for a variety of hydrocarbon precursor gases

 

Similar measurements of the stable gas species were made using different hydrocarbon precursor gases.3.1,3.2 It has been established that regardless of the hydrocarbon precursor gas used, at filament temperatures between 2000 and 2100C the subsequent chemistry is such that methane becomes the dominant hydrocarbon species. Furthermore, at filament temperatures near to, and above, the optimum for diamond growth (ca. 2400C) the relative concentrations of the various stable hydrocarbon species present in the gas mixture, and the way the concentrations vary with temperature, are both remarkably insensitive to the choice of hydrocarbon feedstock gas.

 

4.7 Analysis of the films grown using 0.5% and 1% CH4 in H2

 

Figure 4.5 shows scanning electron micrographs (top view and cross-section) of a polycrystalline diamond film grown on silicon (100) using 0.5% CH4 in H2 under standard deposition conditions (See Section 3.1). Figure 4.6 shows a similar film grown using 1% CH4 in H2 gas mixture. The growth rates (Table 4.3) were calculated from the film thickness, determined from cross-sectional SEM images, divided by the time of growth (6 hours). These deposition experiments were carried to allow the comparison between films grown using standard methane/H2 gas mixtures with those grown using other gas phase systems, such as C/H/Cl, C/H/N and C/H/P systems, following claims that addition of small quantities of these elements has noticeable effects on the growth rate, the morphology and the resulting quality of the diamond films. Full discussion of each of these gas phase systems are given in their respective chapters (5-7).

 

 

Fig.4.5a

Fig.4.5b

Figure 4.5(a). Scanning electron micrograph (SEM) of a polycrystalline diamond film grown on silicon using input gas mixtures of 0.5% CH4 in hydrogen.

Figure 4.5(b). Scanning electron micrograph showing the cross sectional view of the film.

Fig.4.6a

Fig.4.6b

Figure 4.6(a). Scanning electron micrograph (SEM) of a polycrystalline diamond film grown on silicon using input gas mixtures of 1% CH4 in hydrogen.

Figure 4.6(b). Scanning electron micrograph showing the cross sectional view of the film.

 

Gas Mixture

Film Thickness (mm)

after 6 hours growth

Deposition Rate (mm/hr)

 

0.5% CH4 in H2

 

1.9

 

0.32

 

1% CH4 in H2

 

2.4

 

0.40

 

Table 4.3. Diamond film thickness and deposition rate for 0.5% and 1% CH4 in H2 gas mixtures.

 

4.8 Appendix: Experimental Data

 

The following tables show actual experimental data recorded for 1% CH4 in H2. Table 4.4 shows the signal intensities measured for the stable hydrocarbon species as well as CH3 radicals as a function of filament temperature. Table 4.5 shows signal intensities of the stable species after background subtraction and MS sampling efficiency corrections. The mole fractions of these species were then determined by direct room temperature calibration, and are shown in Table 4.6. Methyl radical concentrations were also determined using a slightly different procedure (See Section 3.6 (b) ). Error calculations using ASEASYAS are displayed in the tables for five selected temperature readings.

 


1% CH4 in H2 at 20 Torr vs. Filament Temperature

 

MS Probe Parameters: -6% (DISCRIM), -20% (DELTAM), -40% (RESN), 2550V (SEM), 3.0V (CAGE) 140mA (EMISS).

 

MS Pressure = 8x10-7 Torr

 

Table 4.4. Signal Intensity vs. Filament Temperature

 

Fil. Temp

(C)

CH4

(15.6eV)

C2H2

(15.6eV)

C2H4

(13.6eV)

CH3

(13.6eV)

H2

(15.6eV)

23

 

1470 32

8 2

3 1

0 0

141000

3000

800

 

1008

5

4

6

119000

1400

 

832

6

3

6

108000

1600

 

707 17

6 1

3 1

5 1

100000

4000

1810

 

602

14

7

6

95000

1910

 

549 16

18 3

10 2

5 1

93000

4000

2030

 

520

19

7

6

91000

2120

 

465

20

7

5

88000

2210

 

423 15

21 4

10 2

5 1

87000

3000

2310

 

381

26

11

6

83000

2400

 

305

34

13

9

83000

2490

 

230 9

55 5

15 2

8 1

79000

4000

2575

 

168

67

15

11

77000

2655

 

140

73

18

8

74000

Background Signal

4 1

3 1

1 1

0 0

3560

500


Table 4.5. Signal Intensity vs. Filament Temperature after background subtraction and MS efficiency corrections

 

Fil. Temp

(C)

CH4

(15.6eV)

C2H2

(15.6eV)

C2H4

(13.6eV)

CH3

(13.6eV)

23

 

1466 108

5.0 3

2.0 2

0.0 0

800

 

1195

2.3

3.6

2.2

1400

 

1090

3.9

2.6

3.4

1600

 

1002 98

4.3 4

2.9 3

3.0 2

1810

 

899

16.5

9.0

5.3

1910

 

837 90

23.1 8

13.8 5

4.2 2

2030

 

811

25.1

9.4

6.1

2120

 

750

27.7

9.8

5.0

2210

 

690 73

29.6 10

14.8 5

5.4 2

2310

 

652

39.8

17.3

7.7

2400

 

521

53.6

20.8

13.4

2490

 

412 54

94.7 19

25.5 7

12.9 3

2575

 

307

119.8

26.2

19.3

2655

 

265

136.6

33.2

14.5

 

Room temperature calibration of stable species

 

1% CH4 in H2 at 20 Torr = 1466 c/s 33 (15.6eV)

1% C2H2 in H2 at 20 Torr = 3260 c/s 44 (15.6eV)

1% C2H4 in H2 at 20 Torr = 1131 c/s 30 (13.6eV)

1% CH3 in H2 at 20 Torr = 606 c/s 39 (13.6eV)*


Table 4.6. Species Mole Fractions vs. Filament Temperature

 

Fil. Temp

(C)

CH4

(15.6eV)

C2H2

(15.6eV)

C2H4

(13.6eV)

CH3

(13.6eV)

23

 

1.000

0.098

0.002

0.001

0.002

0.002

0.000

0.001

800

 

0.813

0.001

0.003

0.004

1400

 

0.741

0.001

0.002

0.006

1600

 

0.682

0.083

0.001

0.001

0.003

0.003

0.005

0.003

1810

 

0.611

0.005

0.008

0.009

1910

 

0.570

0.075

0.007

0.003

0.012

0.005

0.007

0.004

2030

 

0.552

0.008

0.008

0.010

2120

 

0.510

0.008

0.009

0.008

2210

 

0.469

0.061

0.009

0.003

0.013

0.005

0.009

0.004

2310

 

0.444

0.012

0.015

0.013

2400

 

0.354

0.016

0.018

0.022

2490

 

0.280

0.043

0.029

0.006

0.023

0.007

0.021

0.006

2575

 

0.209

0.037

0.023

0.032

2655

 

0.181

0.042

0.029

0.024

 

* The error for 1% CH3 signal may be larger than the value quoted here since there will be errors arising from the determination of the beam component of the sampled gas (~0.35 of the total signal). See Section 3.5 (i) for details of the measurements.